Certain 2-(2'3'4'trisubstituted benzoyl)-1,3-cyclohexanedione herbicides are described in U.S. Pat. Nos. 4,780,127, issued Oct. 25, 1988; 4,816,066, issued Mar. 28, 1989; and PCT International Publication No. WO 90/05712, published May 31, 1990, and entitled Certain 2-(2',3',4'-trisubstituted benzoyl)-1,3-cyclohexanediones, with William J. Michaely, inventor and all incorporated herein by reference.
The above-described herbicidal compounds can have the following structural formula ##STR2## wherein R.sup.1 is hydrogen, halogen, nitro, C.sub.1 -C.sub.2 alkyl, C.sub.1 -C.sub.2 alkoxy or trifluoromethyl; R.sup.7 through R.sup.12 are hydrogen or C.sub.1 -C.sub.4 alkyl or R.sup.7, R.sup.8, R.sup.11 and R.sup.12 are methyl and R.sup.9 and R.sup.10 together is carbonyl; R.sup.2 is hydrogen, halogen, C.sub.1 -C.sub.4 alkoxy, C.sub.1 -C.sub.4 alkyl --OCH.sub.2 CH.sub.2 OCH.sub.3, --OCH.sub.2 CH.sub.2 OC.sub.2 H.sub.5, or a substituted alkyl preferably C.sub.1 -C.sub.4 alkoxy, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 haloalkyl, --CH.sub.2 CH.sub.2 OCH.sub.3,--CH.sub.2 CH.sub.2 OC.sub.5, or C.sub.1 -C.sub.2 alkoxy methylene; and R.sup.3 is C.sub.1 -C.sub.4 alkyl or C.sub.1 -C.sub.4 haloalkyl.
These herbicides can be prepared by reacting a dione of the structural formula ##STR3## wherein R.sup.7 through R.sup.12 are as defined with a mole of a trisubstituted benzoyl chloride compound of the structural formula ##STR4## wherein R.sup.1, R.sup.2 and R.sup.3 are as defined above.
The process of the present invention relates to the preparation of precursor trisubstituted benzoic acid intermediates that are easily converted to the above described trisubstituted benzoyl chloride compounds.
U.S. Pat. No. 4,780,127 teaches the preparation of 2,3 disubstituted-4-(alkylsulfonyl) benzoic acid by the oxidation of the corresponding 2,3-disubstituted-4-(alkylthio) benzoic acid with an oxidizing agent such as sodium hypochlorite or m-chloroperbenzoic acid in a solvent such as methylene chloride. Hydrogen peroxide is not specifically recited as an oxidizing agent.
However, hydrogen peroxide (H.sub.2 O.sub.2) is a well known oxidizing agent for converting aliphatic or aromatic sulfides to sulfoxides. Customarily, the hydrogen peroxide is used with an equal mole amount of formic acid which reacts with the hydrogen peroxide to form the more active performic acid oxidizing agent. However, the oxidation reaction yields formic acid which is an undesireable reaction product for the process of this invention.
Hydrogen peroxide, for safety reasons, normally is diluted with water to concentrations of about 30 or 50 percent by weight when used as an oxidizing agent. When these 30 or 50 percent hydrogen peroxide solutions are used as an oxidizing agent for sulfides the reaction is very slow at normal temperatures (20.degree. C.-100.degree. C.).
When an aqueous hydrogen peroxide solution at high concentrations (above 90 percent by weight) is used as the oxidizing agent for converting sulfides to sulfones, the oxidation is initially successful but quickly becomes inefficient.
The reason for this is that the normal reaction mechanism is that one mole of hydrogen peroxide first will oxidize the sulfide reactant to the corresponding sulfoxide with the creation of one mole of water according to the following equation: EQU R'--S--R"+H.sub.2 O.sub.2 .fwdarw.R'--S(O)--R"=H.sub.2 O
It can be seen that as the oxidation reaction proceeds, the concentration of the aqueous hydrogen peroxide solution is increasingly diluted with additional water that is created by the reaction and increasingly the hydrogen peroxide becomes less effective as an oxidizing agents as the concentration of the aqueous solution of hydrogen peroxide becomes less and less.